cs193p – Assignment #6 Part #5

Please note, this blog entry is from a previous course. You might want to check out the current one.

Photo Downloads using Background Fetch

Assignment #6 Task #7

Fetch the URLforRecentGeoreferencedPhotos from Flickr periodically (a few times an hour when your application is in the foreground and whenever the system will allow when it is in the background using the background fetching API in iOS). You must load this data into a Core Data database with whatever schema you feel you need to do the rest of this assignment.


To set up a background fetch, you only need to tell the app what to do when it gets called while it is in the background. In our case: load photos. If the download works, do something with the photos then call the provided completion handler, letting it know that there is new data. If it fails call the completion handler and tell it that the download did not work:

- (void)application:(UIApplication *)application
performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
    [FlickrHelper loadRecentPhotosOnCompletion:^(NSArray *photos, NSError *error) {
        if (error) {
            NSLog(@"Flickr background fetch failed: %@", error.localizedDescription);
            completionHandler(UIBackgroundFetchResultFailed);
        } else {
            NSLog(@"%d photos fetched", [photos count]);
            completionHandler(UIBackgroundFetchResultNewData);
        }
    }];
}

Note that I use the ephemeral download from part #2 instead of the background download from part #3. When this method gets called the app is already in the background. Starting background downloads would mean that the device can decide when to actually start those downloads … probably never …

However, because we want to be “good background citizens”, we adjust the helper function to do not start downloads when there is only a cellular network available, and we limit the download timeout:

#define BACKGROUND_FLICKR_FETCH_TIMEOUT 10
+ (void)loadRecentPhotosOnCompletion:(void (^)(NSArray *places, NSError *error))completionHandler
{
    ...
    config.allowsCellularAccess = NO;
    config.timeoutIntervalForRequest = BACKGROUND_FLICKR_FETCH_TIMEOUT;
    ...
}

… but we need to tell the app, that we would like to get called as often as possible, if it is convenient (for the device):

- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    ...
    [[UIApplication sharedApplication] setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
    ...
}

Testing this part is possible using Xcode via Debug -> Simulate Background Fetch. Or you can create a new scheme with background fetch enabled during startup:

The complete code for tasks #1 to #7 is available on github.